In mechanical wood processing industry a need arises, e.g., in saw mills and manufacturing processes of wood-based boards for an apparatus capable of length screening of particles mostly comprising elongated wood particles. Such raw materials include various sawing residues, chipped veneer residues from plywood manufacture, as well as the relatively large OSB (oriented structural board) and waferboard slices typically employed in the North-American wood-based particleboard production.
Chipped veneer residues obtained from plywood manufacture contain particles often deviating from the normal chip length by being comprising of vastly overlength veneer splinters that must be screened away from the fraction of normal size chips to permit successful further processing of the veneer residue material.
To optimize the properties of OSBs and waferboards as to, e.g., their flexural strength, advantageously the shortest slices obtained from the slicing machine are screened entirely away from the process and the fraction of remaining short slices is chiefly used in laying the core of the board, while the fraction of longer slices is laid on the surface layers. To implement such fractionation, an effective method of length screening of the slice stock is required.
Currently, length screening is chiefly attempted by using planar, drum or disk screens giving, however, unsatisfactory screening results. The screen openings of planar and drum screens easily permit the passage of overlong particles to the wrong side of the screen if the particles are sufficiently narrow and thin to negotiate the screen openings.
In a disk screen, the shafts driving the disks have a relatively small diameter, whereby the shafts cannot contribute to the actual screening process. Hence, when length screening is attempted by means of a disk screen, the design of such a screen is based on the assumption that the longer wood particles would also be thicker. This assumption is not universally valid, however, because particles of relatively large length may still be very thin: for instance, long veneer splinters or thin OSB slices made with a slicing machine are unsuitable for sufficiently effective length screening with a disk screen which is actually developed for thickness screening.
EP patent application no 89630079.5 (publication no. 0340148) discloses a screening apparatus for chip-like materials, said apparatus being formed by adjacently mounted elongated rolls on which are arranged stop rings with a slightly larger diameter than that of the roll, said stop rings being spaced from each other at a distance. Such a construction offers a reasonably good function, yet at a relatively high cost.